1. Field of the Invention
This invention relates to electrical connectors for making electrical connections to printed circuit boards. More particularly, this invention relates to zero or low insertion force electrical connectors for making electrical connections to conductive strips mounted along the edges of printed circuit boards.
2. Prior Art
There are many types of electrical connectors in the prior art for making electrical connections to conductive strips dispersed along opposing sides on the elongated edge of a printed circuit board. One such type is called a "zero insertion force" connector, which allows a circuit board to be inserted into the connector without any substantial insertion force. The board is thus inserted into the connector to make an electrical connection without any wiping and potentially harmful friction forces against the delicate electrical contacts on the opposing sides of the edge of the board.
Examples of zero insertion force connectors are disclosed in U.S. Pat. Nos. 4,575,172 and 3,848,952. These connectors generally consist of a housing enclosing a pair of generally C-shaped or U-shaped electrical contact arms of differing lengths. The edge of the printed circuit board is inserted between the arms without making contact with either arm. The board is then rotated into position in the housing so that one arm engages one side of the edge of the board and the second arm engages the opposing side of the edge of the board. At least one arm thus engages a contact on the edge of the board with sufficient normal force to assure electrical contact, but without any wiping between the arm and the contact.
One problem with certain of the older prior art connectors of the types shown in these patents is that they require interference mounting of the contacts in passages in the base of the connector. Support members for the contacts penetrate and interference fit within the connector mounting passages in the base of the connector housing to mount the contacts in the housing. When, as is common, a large number of contacts are mounted in a housing, the cumulative stress of the contact/passage interference fits can axially bow the connector housing, especially when the housing is heated during the end user application process. An axially-bowed housing is more difficult to mount on a circuit board since the center of the housing tends to bow away from the planar circuit board surface in which it is mounted. The connector may thus be mechanically as well as electrically unstable on the printed circuit board on which it is mounted.
Another problem with the prior art connectors is that of mounting the C- of U-shaped contact centered in the housing to provide the appropriate clearance between the contact arms and the housing. The C- and U-shaped contacts in the patents described above, for example, require precise location of the contacts in mounting passages in the housing in order to mount the contacts in the connector housing and attain appropriate clearance. In addition, as noted above, the contacts have support members that must fully penetrate and grip mounting passages to retain the contacts in position in the housing. These prior art contacts therefore require precise assembly techniques and relatively complicated structure to assemble and maintain the contact in the proper horizontal and vertical orientation in the housing.
Also, in many of these prior art connectors, the contacts are exposed to direct impact against the edge of a printed circuit board when the board is inserted out of alignment into the connector or when the board edge is warped to a significant degree. This direct impact can damage the contacts in the connector as well as the contacts on the edge of the board.
A still further problem with the prior art connector arises in the molding process. In order to minimize the size of a connector, the design should minimize the thickness of both the contacts and the wafer walls that maintain the spacing between the contacts. This creates a molding problem, since it is difficult if not impossible to structure a mold projection that will reliably mold a series of very thin contact spaces between very thin wafers.